Greatest, being subjective, means there is technically no right or wrong answer since one person’s definition of greatest is the same as someone else.
But I would say Britain is the greatest nation on earth. The reason is that the agricultural revolution and industrial revolution started there, and without them the world we live in wouldn’t be possible. The age of reason was also partially British. Great British thinkers include Darwin and Newton.
Britain stood alone against the Nazis for a few years before Russia got involved in WW2.
Plus many of the nations today who are pioneering science, technology and medicine were heavily influenced by Britain. Places like Canada, the US, Ireland, New Zealand, Australia, Israel & India. Roughly 1/3 of all people who live in OECD nations either are British or live in nations heavily influenced by Britain.
So I’d consider Britain the greatest nation on earth (I’m from the US). Many of the advances that made the world we live in today possible came from Britain or nations heavily influenced by Britain.